1. Field of the Invention
The present invention relates to a semiconductor, and more particularly to an improved semiconductor device and fabrication method thereof for forming a guard ring between n-wells provided for fabricating a peripheral circuitry adjacent to an input protection transistor, thereby stabilizing a current-voltage operation characteristic of an input pad.
2. Description of the Background Art
Among semiconductor devices, a DRAM (Dynamic Random Access Memory) refers to a memory device wherein a basic cell is provided with a selection transistor and a capacitor. A gate of the selection transistor is connected to a word line, a drain thereof is connected to a bit line, and a source thereof is connected to an electrode or the capacitor, that is, to an earthed electrode. Such a DRAM stores therein data depending upon whether there is an electrical charge loaded in the capacitor. The DRAM developed in the 1970's is still being studied for its larger integration.
FIG. 1 is a cross-sectional view illustrating a semiconductor device with regard to an input pad peripheral circuit according to a background art. As shown therein, a plurality of device isolation regions a, b, c are formed in the upper surface of a semiconductor substrate 10. A first and second n-wells 20, 30 are formed adjacent to the device isolation regions a, b in the semiconductor substrate 10. The first and second n-wells 20, 30 are isolated by the device isolation region b. In the first n-well 20 there is formed an n+ diffusion layer 21 which is connected to a first supply voltage Vdd. In the second n-well 30 there are formed an p+ diffusion layer 31 which is connected to first supply voltage Vdd, and a p+ diffusion layer 32 which is connected to a second supply voltage Vcc.
A p-channel transistor Q2 having the p+ diffusion layers 31, 32 as drain and source and having a gate electrode G2 is formed in the second n-well 30. The first supply voltage Vdd denotes an internal supply voltage generated from an externally applied supply voltage Vcc and is generally lower than the externally applied supply voltage Vcc. To the left side from the first n-well 20, there is provided an input protection transistor Q1 adjacent to the device isolation region a. The input protection transistor Q1 is an n-channel transistor. The n-channel transistor Q1 includes the gate electrode G1 formed in the upper surface of the semiconductor substrate 10, and the n+ diffusion layers 11, 12 as source and drain, respectively. The gate electrode of the input protection transistor Q1 and the n+ diffusion layer 12 are connected to a ground voltage Vss serving as a third supply voltage. The n+ diffusion layer 11 is connected to an input voltage Vin which is received from an input pad (not shown). An n+ diffusion layer 13 is horizontally spaced from the left side of the input protection transistor Q1 and adjacent to the device isolation region c, and it is connected to the third supply voltage. A horizontal npn parasite bipolar transistor Q3 is formed by the n+ diffusion layer 11 connected to the input voltage Vin of the input protection transistor Q1 and by the n+ diffusion layer 13 connected to the third supply voltage Vss.
Voltage V.sub.GD connected to the gate electrode G2 of the p-channel transistor Q2 denotes a voltage applied to the gate of the p-channel transistor Q2 and a voltage of 0V is applied thereto. Here, voltage V.sub.BB denotes a back bias voltage.
In the input pad peripheral circuitry as shown in FIG. 1, when the input voltage Vin is less than a threshold voltage Vth, the input protection transistor Q1 is turned on, thereby allowing current to flow to an input voltage Vin of the input protection transistor Q1.
Here, when a temperature goes up, electron-hole pairs are generated from a depletion region provided between the first n-well 20 and the p-type semiconductor substrate 10, whereby holes h+ flow toward the p-type semiconductor substrate 10, and electrons e- flow toward the first n-well 20.
At this time, the first n-well is in a floating state so that the received electrons e- serve to lower the voltage, whereby a forward voltage is applied between the first n-well 20 and the p-type semiconductor substrate 10. Accordingly, electrons are injected toward the p-type semiconductor substrate 10.
When electrons are injected into the p-type semiconductor substrate 10, the injected electrons are turned to a base current of parasite bipolar transistor Q3, and the current amplified as much as the gate of the transistor is applied between n+ region 11 to which is applied Vin and n+ region 13 which is connected to Vss.
As a result, when the first n-well 20 and the second n-well 30 are biased by Vdd and an open/short test is carried out, a stable current-voltage characteristic is not realized around the input pad peripheral circuitry.
That is, the input pad peripheral circuitry of the conventional DRAM device according to the conventional art has several disadvantages: firstly, because the input protection transistor Q1 is activated by a thermal carrier generated in a high temperature, there is a strong possibility of incurring an operational error in the DRAM device; and secondly, when carrying out an open/short test by biasing the n-well regions 20, 30 to the first supply voltage Vdd, the n-well regions respectively remain in a floating state, thereby incurring a device error in a high possibility.